The present invention relates generally to subterranean well construction, and more particularly to plugs, plug systems, and methods for using these plugs and systems in subterranean wells.
Cementing operations may be conducted in a subterranean formation for many reasons. For instance, after (or, in some cases, during) the drilling of a well bore within a subterranean formation, pipe strings such as casings and liners are often cemented in the well bore. This usually occurs by pumping a cement composition into an annular space between the walls of the well bore and the exterior surface of the pipe string disposed therein. Generally, the cement composition is pumped down into the well bore through the pipe string, and up into the annular space. Prior to the placement of the cement composition into the well bore, the well bore is usually full of fluid, e.g., a drilling fluid. Oftentimes, an apparatus known as a cementing plug may be employed and placed in the fluid ahead of the cement composition to separate the cement composition from the well fluid as the cement slurry is placed in the well bore, and to wipe fluid from the inner surface of the pipe string while the cementing plug travels through it. Once placed in the annular space, the cement composition is permitted to set therein, thereby forming an annular sheath of hardened substantially impermeable cement therein that substantially supports and positions the pipe string in the well bore and bonds the exterior surface of the pipe string to the walls of the well bore.
In some circumstances, a pipe string will be placed within the well bore by a process comprising the attachment of the pipe string to a tool (often referred to as a “casing hanger and running tool” or a “work string”) that may be manipulated within the well bore to suspend the pipe string in a desired location, including, but not limited to, suspension at or below the sea floor in off-shore operations. In addition to the pipe string, a sub-surface release cementing plug system comprising a plurality of cementing plugs may also be attached to the casing hanger and running tool. Such cementing plugs may be selectively released from the running tool at desired times during the cementing process. The sub-surface release cementing plug system may comprise a bypass mechanism that permits fluids to flow through the plugs at appropriate times. Conventional bypass mechanisms may comprise, for example, a rupture disk, which when punctured, may permit some degree of flow through the plug system. Additionally, a check valve, typically called a float valve, will be installed near the bottom of the pipe string. The float valve may permit the flow of fluids through the bottom of the pipe string into the annulus, but not the reverse. A cementing plug will not pass through the float valve. When a first cementing plug (often called a “bottom plug”) is deployed from a sub-surface release cementing plug system and arrives at the float valve, fluid flow through the float valve is stopped. Continued pumping results in a pressure increase in the fluids in the pipe string, which indicates that the leading edge of the cement composition has reached the float valve and activates a by-pass mechanism built into the bottom plug. After the bottom plug has been opened, the cement composition flows through the float valve and into the annulus. When the top plug contacts the bottom plug which had previously contacted the float valve, fluid flow is again interrupted, and the resulting pressure increase indicates that all of the cement composition has passed through the float valve. It is important that all of the desired cement composition be pumped into the annulus from the pipe string. If not, the cement remaining in the pipe string will have to be drilled out before any further activities can take place. Furthermore, the annulus might not be properly filled with cement, and undesirable formation-fluid migration or failure of the pipe string may result. On the other hand, if the cement is overdisplaced, a lower portion of the annulus might not be properly filled with cement, and undesirable formation-fluid migration or failure of the pipe string could result. Overdisplacement of the cement is considered a worse problem than underdisplacement, as it can be more difficult to correct.
Sub-surface release cementing plug systems often have a number of difficulties. For example, a sub-surface release cementing plug system may be damaged when weight is transferred to it while it is being attached to the running tool and/or being inserted into the top of the casing. Such weight transfer may shear the bypass mechanism present in the bottom cementing plug; in such circumstance operations may be performed by removing the bottom plug and continuing the operation by relying solely on the top plug. Another problem is that conventional bypass mechanisms-when activated-may overly restrict the flow of a desired fluid through the cementing plugs. Flow restrictions are problematic because they may generate hydraulic ram effects against subterranean formations intersected by the borehole while the pipe string is being installed, which may result in complications such as hydraulic fracturing of the subterranean formation, for example, which may lead to problems such as lost circulation, differential sticking of the pipe string against the bore hole, loss of well control, difficulty or inability to place a cement composition at a desired location in the annular space, and other problems. Difficulties may also be encountered in releasing the plug sets in a timely and accurate fashion, to ensure that the bottom cementing plug is released in spacer fluid ahead of the leading edge of the cement slurry. The timely and accurate release of cementing plugs via a free fall device (e.g., weighted plastic balls) is particularly difficult in deep wells where the fluid capacity of the drill string may range up to about several hundred barrels. One attempt at solving this problem has been to use a cementing plug system wherein the bottom plug is released by the use of a positive displacement device, e.g., a drill pipe dart. However, this method has been problematic because the dart is captivated within the cementing plug once the plug has landed on the uppermost float valve near the bottom of the well bore and the bypass system has been activated, which may increase the length of the bottom plug and may restrict the flow rate through the bypass mechanism.
Cementing plugs must be drilled out of the casing when the cementing operation has been completed. For this reason, the plugs are usually made from materials that are easily drilled. Such materials include some kinds of plastic, aluminum, cast iron, and others. Although generally speaking plastic materials are easier to drill out than metal materials, they generally are subject to rapid erosion when exposed to conditions in the well.
Personnel conducting cementing operations often encounter a further problem in attempting to accurately determine the volume of the casing string prior to preparing the cement composition or to deploying a final (“top”) cementing plug. This problem is typically caused by the fact that casing capacity tables are based upon nominal casing inner diameters for a given casing size and weight. Actual casing inner diameters often tend to be slightly larger than these published nominal inner diameters. Accordingly, on long casing strings the actual casing displacement can be significantly larger than the calculated theoretical volume, which may inhibit operators from displacing the final cementing plug to its desired shut-off point—e.g., from reaching and contacting the preceding cementing plugs atop the uppermost float valve near the bottom of the casing. This often prevents the customer from conducting a casing integrity test at the completion of cementing operations, and may result in extended drill out times due to excessive volumes of cement remaining inside the casing.
An additional problem often encountered with conventional cementing operations relates to the conventional configuration of float valves typically installed at the leading end of casing installed in a well bore. Typically, such float valves have an opening that is relatively small in relation to the inner diameter of the casing. In certain circumstances wherein the casing is disposed horizontally, such as when the casing is installed in a horizontal well, for example, sediment may accumulate along the bottom of the horizontally disposed casing. When a bottom cementing plug is displaced through the well bore, the plug may encounter an amount of sediment that is sufficient to slow the cementing plug's velocity and stop the cementing plug short of landing against the float valve and sealing against the entire diameter of the casing. This is problematic because the failure of the cementing plug to seal prevents operations personnel from conducting a pressure test on the casing. Furthermore, the problem becomes increasingly problematic as casing diameter increases, because a greater amount of sediment may accumulate due to factors such as decreased fluid velocities (which may permit debris to fall out of suspension) for a given rate of circulation, and because the relatively small inner diameter of conventional float valves in relation to the casing diameter forces the bottom cementing plug to displace the sediment to a greater height in order to propel it through the inner diameter of the float valve, when the casing is disposed horizontally. Sediment may build in front of the bottom plug until the pressure differential required to sustain plug movement exceeds the “opening” pressure of the plug (e.g., the pressure at which the bypass mechanism is activated). At this time cement flow will be established through the plug and over the top of the horizontal, accumulated sediment bed resident between the bottom plug and the upper float valve. When the top cementing plug at the tail of the cement slurry is displaced to the bottom plug, both plugs will continue to displace and push the cement and sediment ahead of the plugs until such time as the compacted sediment prevents the plugs from achieving sealing contact with the upper float valve. The inability of the cementing plugs to establish sealing contact with the float valve will prevent achievement of a pressure shut-off. Accordingly, contaminated cement and sediment may fill the remaining casing below the upper float and/or pass around the end of the casing string, thereby producing what is often referred to as a “wet shoe.” Operators will have no surface indication that the plugs have failed to displace all debris through the float valve, because the landing pressure of the top plug will generally be much greater than the activation pressure of the bottom plug by-pass mechanism. Accordingly, the only indication that a problem exists may be the failure to properly land the top plug, along with the resulting “soft drill out” and/or the failure to achieve an acceptable shoe test after drill out.